Overview

Using HEC-RAS to perform 2D modeling or combined 1D/2D modeling is very easy and straight forward. The following are the basic steps for performing 2D (or combined 1D/2D) modeling within HEC-RAS:

  1. Establish a Horizontal Coordinate Projection to use for your model, from within HEC-RAS Mapper. This is normally done by selecting an existing projection file from an ESRI shapefile or another mapping layer.
  2. Develop a terrain model in HEC-RAS Mapper. The terrain model is a requirement for 2D modeling, as it is used to establish the geometric and hydraulic properties of the 2D cells and cell faces. A terrain model is also need in order to perform any inundation mapping in HEC-RAS Mapper.
  3. Build a Manning's n layer data set within HEC-RAS Mapper, using land cover data layers and polygon layers, in order to establish Manning's n values within the 2D Flow Areas. Additionally, HEC-RAS has an option for user defined polygons that can be used to override the base Manning's n values. These user defined polygons could be used as calibration zones.
  4. Add any additional mapping layers that may be needed for visualization, such as aerial photography, levee locations, road networks, etc.
  5. From within the HEC-RAS Mapper draw a boundary polygon for each of the 2D Flow Areas to be modeled. Or you can import the X, Y boundary coordinates from another source.
  6. Layout any break lines within the 2D flow area to represent significant barriers to flow, such as: levees, roads, natural embankments, high ground between main channel and overbank areas, hydraulic structures, etc.
  7. Using the 2D Flow Area editor, create the 2D computational mesh for each 2D Flow Area.
  8. Edit the 2D Flow Area mesh in order to improve it, such as: add additional break lines; use the refinement regions option to increase or decrease cell density as needed; Add, Move, or Delete cell centers where needed. Use the mesh refinement region tool to make a nice channel mesh.
  9. Add any internal hydraulic structures or bridges inside of the 2D flow area(s) using the SA/2D Area Hydraulic Connection feature.
  10. Run the 2D geometric pre-processor from RAS Mapper in order to create the cell and face hydraulic property tables.
  11. Connect the 2D Flow Areas to 1D Hydraulic elements (river reaches, Lateral structures, storage area/2D flow area hydraulic connections) as needed.
  12. From the RAS Mapper or the Geometric Data editor, draw any external boundary condition lines along the perimeter of the 2D flow areas.
  13. Enter all of the necessary boundary and initial condition data for the 2D flow areas in the Unsteady Flow data editor.
  14. From the Unsteady Flow Simulation window, set any necessary computational options and settings needed for the 2D flow areas.
  15. Run the Unsteady flow simulation.
  16. Review the combined 1D/2D output in RAS Mapper, as well as using the existing output capabilities for the 1D portions of the model.